This invention relates to an electrophotographic printing machine and more particularly to a method and apparatus for removing toner dispersant from an image formed from a liquid developer.
A typical electrostatographic printing machine employs an imaging member that is exposed to an image to be printed. Exposure of the imaging member records an electrostatic latent image on it corresponding to the informational areas contained within the image to be printed. The latent image is developed by bringing a developer material into contact therewith. The developed image recorded on the photoconductive member is transferred to a copy sheet such as paper, either directly or via an intermediate transport member. The developed image on the copy sheet is generally subjected to heat and/or pressure to permanently fuse it thereto.
Two types of developer materials are typically employed in electrostatographic printing machines. One type of developer material is known as dry developer material and comprises toner particles or carrier granules having toner particles adhering triboelectrically thereto. Another type of developer material is a liquid material comprising a liquid carrier or dispersant having toner particles dispersed therein.
Liquid developer typically contains about 2 percent by weight of fine solid particulate toner material dispersed in the liquid carrier. The liquid carrier is typically a hydrocarbon. In the developing process, the developed image on the photoreceptor contains about 12 weight percent of particulate toner in liquid hydrocarbon carrier. To improve the quality of transfer of the developed image to a receiving member or copy sheet, the image should be conditioned, i.e. percent solids in liquid should be increased by removing liquid carrier from the image while preventing toner particles from departing the image. Depending on the particular liquid carrier structural composition and its respective properties during the image formation process, e.g. vapor pressure rate, evaporation rate and volatility, the percentage of solids in the liquid should be increased to in the range of 25 to 75 percent. Increase in percent solids may be achieved by removing excess liquid carrier with a porous blotter in the form of a roller or belt (hereinafter collectively referred to as "roller"), typically positioned with respect to the photoconductive member retaining the latent image. When the developed image is transferred to an intermediate belt before final transfer to a final copy sheet, the developed image on the intermediate belt should again be blotted to further increase the percentage of toner solids, so that the amount of liquid on the final copy sheet is minimized, and a well defined, high quality image is produced.
Polymers such as various polyurethanes, olefins, tetrafluoroethylene, and various elastomers, may be processed into open cell poromeric foam material appropriate for use in blotter roller applications, using the teachings, for example, in U.S. Pat. Nos. 3,696,180; 3,729,536; 3,860,680; 3,968,292; 4,157,424, and other methods known in the art. Conductive fillers, organic and inorganic, ionic or electronic may be added to regulate the poromeric material conductivity.
These open cell poromeric forming processes generally produce a top and bottom skin layer. This skin covering generally tends to have pores of a smaller size and a lesser quantity than the open cell inner foam material, and serves as a structural support and protective covering for the inner foam layer. The skin covering must have a porosity sufficient to absorb liquid carrier from the developed image, however, should have a smooth, glossy surface texture so that toner particles from the developed image are prevented from departing the image and embedding into the irregularities found on a more textured blotter roller skin surface. It is important to prevent toner particles from entering the blotter roller, as the pores of the roller become blocked and the rate of absorption of the liquid carrier from the developed image is slowed. Image quality is impaired when toner particles depart therefrom, and frequent cleaning and/or replacement of the roller is necessitated. In addition to a smooth surface on the blotter roller skin covering, a bias applied to the blotter roller having the same polarity as the toner particles of the developed image presents a repelling force between the toner and the roller, further preventing toner particles from entering the blotter roller.
A shortcoming of this type of blotter roller, however, is that the skin covering tends to retard fluid flow through the poromeric material, even in systems having a vacuum assist for removal of the liquid from the roller. Therefore, blotter rollers are often unable to achieve the desired toner solid weight percentage at the required process speed for a high volume production color printer. Process speed and/or image quality have been restricted by prior art blotting devices used in liquid developing systems. This shortcoming is even more apparent due to the increased tendency to use liquid carriers that are less volatile, for environmental, health and safety reasons. Less volatile liquid carriers have a decreased vapor pressure rate, and thereby give off less offensive odors and expose less vapor into the atmosphere during the image forming process. Pollution and potential health risks to individuals working near the machine are thereby reduced. However, due to the lower evaporation rates of these liquid carriers, there is provided yet an increased need for a system with an increased capacity for liquid carrier absorption from the developed image.
The following references may be relevant to various aspects of the present invention.
U.S. Pat. No. 3,955,533 to Smith et al. discloses a squeegee roller system for removing excess developer liquid from the developer-image-bearing photoconductive surface of a drum or the like. The squeegee roller has a covering of a predetermined hardness which is biased with a predetermined force against the moving surface carrying a developed image to cause excess developer to flow into a receptacle.
U.S. Pat. No. 4,263,391 to Saito et al. discloses an elastic rotary member used in the developing process, composed of an electroconductive rigid core member, an electroconductive porous elastic member, and a liquid permeable insulating flexible member composed of a netting or a material having perforations. The outer member is designed to allow the elastic rotary member, having liquid developer material absorbed therethrough, to squeeze out such developer material for deposit onto the latent image carrying member when the elastic rotary member is compressed thereagainst, and then to recover excess liquid developer upon recovery of the elastic rotary member from the compressed state after leaving contact with the latent image carrying member.
U.S. Pat. No. 4,258,115 to Magome et al. similarly describes a developing elastic roller member having a rigid shaft and an elastic foam member surrounding the shaft, composed of a netting or apertured sleeve. The elastic roller supplies developing liquid from the roller to an electrostatic image bearing surface when urged against such surface. The elastic roller then collects excess developing liquid after having been in contact with the image bearing surface, and finally deposits the excess developing liquid in a liquid pool after the elastic roller is urged against a refresh roller.
U.S. Pat. No. 4,299,902 to Soma et al. discloses an image forming process which utilizes an elastic roller or belt having a flexible outer member composed of a netting or a material having perforations. The elastic roller or belt applies liquid developer to a latent image and then absorbs excess liquid developer. Part of the roller or belt is kept in a liquid developer reservoir.
U.S. Pat. No. 4,985,733 to Kurotori et al. describes an image fixing unit for a wet-type electrophotographic copying machine comprises a blotter roller which absorbs a carrier liquid component, such as Isopar, contained in a developer deposited imagewise by an image-transfer charger on a transfer sheet. The blotter roller is comprised of a porous covering material such as a non-woven fabric or cotton and an elastic material such as silicone rubber. The transfer sheet is transported into an image fixing unit wherein a pressure-application roller is in contact with a heat-application roller, such that a toner image formed on the transfer sheet is thermally fixed thereto, while the transfer sheet passes through a nip between the heat-application roller and the pressure-application roller. The blotter roller is in contact with a back-up roller with a predetermined pressure. A cleaning pad, brush or roller may be used to clean the blotter roller of toner particles adhering thereto.
U.S. Pat. No. 4,286,039 to Landa et al discloses an image forming apparatus comprising a deformable polyurethane roller, which may be a squeegee roller or blotting roller which is biased by a potential having a sign the same as the sign of the charged toner particles in a liquid developer. The bias on the polyurethane roller is such that it prevents streaking, smearing, tailing or distortion of the developed electrostatic image and removes much of the liquid carrier of the liquid developer from the surface of the photoconductor.
U.S. Pat. No. 4,392,742 to Landa discloses a cleaning system for a liquid developer electrostatographic copier comprising a roller formed with a resilient material, such as a closed-cell elastomer, having externally exposed, internally isolated surface cells. During an operation, residual toner and excess liquid on an imaging surface is absorbed by the cleaning roller. The cleaning roller is then compressed to squeeze out liquid from the roller, leaving the roller dry.
U.S. Pat. No. 5,028,964 to Landa et al. discloses an apparatus for image transfer which comprises an intermediate transfer member and a squeegee for removing excess liquid from the toner image prior to transferring an image. The intermediate transfer member is operative for receiving the toner image therefrom and for transferring the toner image to a receiving substrate. Transfer of the image to the intermediate transfer member is aided by providing electrification of the intermediate transfer member to a voltage having the same bias as that of the charge particles.
U.S. Pat. No. 4,607,947 to Ensing et al. discloses a circulating cleaning member comprising a multiplicity of spaced-apart openings or perforations. A surface of the cleaning member collects residues or toner from a surface to be cleaned.
Copending continuing application for U.S. patent Ser. No. 08/082,141, filed Apr. 12, 1993, and having a common assignee as the present application, discloses a device and method to transfer an image to a nonabsorbing intermediate after low pressure blotting or dispersant absorption by means of a belt, whereby the belt is passed over a bias pressure roller that applies a bias to the belt to repel toner particles therefrom, and a backup roller that removes the dispersant from the belt in conjunction with a squeeze roller.
Copending application for U.S. patent Ser. No. 08/107,876, filed Aug. 8, 1993, and having a common assignee as the present application, discloses a roller for removal of excess carrier liquid from a liquid developed image, comprising a rigid porous electroconductive supportive core, a conformable microporous foam material provided around the core, and a pressure controller for providing a positive or negative pressure to the roller.
Copending continuing application for U.S. patent Ser. No. 08/017,453, filed Feb. 12, 1993, and having a common assignee as the present application, discloses a porous roller for increasing the solids content of an image formed from a liquid developer. The liquid dispersant absorbed through the roller is vacuumed out through a central cavity of the roller.